Photographic emulsion



United States Patent PHOTOGRAPHIC EMULSION Richard C. Herold, West Chester, Pa., assignor to 'Aeroprojects Incorporated, West Chester, Pa., a corporation of Pennsylvania No Drawing. Filed Sept. 13, 1955, Ser. No. 534,163

5 Claims. (Cl. 96-108) This invention relates'to photographic emulsions and more particularly to silver halide emulsions of increased speed, with good contrast, relatively low fog level, and good stability.

It is known that the sensitivity and speed of photographic emulsionsmay be increased by bathing the emulsion in water or ammonia solutions and extending the time of digestion or ripening. Recently, considerable interest has developed in increasing sensitivity and speed of such emulsions by the addition of chemical sensitizers.

Chemical sensitizers may be divided into two general classes. The first of these requires a long digestion time with the sensitizers in intimate contact with the silver halide under proper conditions of time and temperature. Examples of this type are those compounds usually found naturally in photographically-active gelatins, generally understood to be sulphur-bearing compounds, such as allyl thiourea. The second type of these sensitizers includes those which do not require a long digestion time at specific temperatures to achieve their sensitizing effect. They require only an intimate mixing with the silver halide emulsion. Optical sensitizers, including a large number of dyes, and some metallic sensitizers, such as gold and gold compounds, ruthenium, rhodium, palladium, osmium, indium, platium, tin, iron salts, mercury, chloropalladites, chloroplatinates, copper, silver, and thallium, are of this type.

Most of the compounds used to enhance the sensitivity of photographic emulsions usually involve some loss of other important emulsion properties, such as fog level, contrast, and stability.

It has been discovered'that the sensitivity and speed of photographic silver halide emulsions may be increased with little or none of the excess fog which often results from the use of chemical sensitizers by incorporating rubidium and cesium in the form of their water soluble salts in the emulsions. Rubidium and cesium would not be expected to possess such sensitizing properties in'view of their position in group l-A of the periodic table.

It is an object of this invention to provide silver halide emulsions with increased speed, good contrast, and relatively low fog level.

It is another object of this invention to provide for sensitizing photographic emulsions with rubidium and cesium salts.

It is still another object of this inventionto provide methods of making photographic emulsions with increased light sensitivity.

,It is a still further object'of this invention to provide photographic emulsions having desirable properties by using the water-soluble salts of rubidium and cesium.

Metals previously used as sensitizers are generally found in the group of transition elements, i.e., those having incomplete electronic subshells. Rubidium and cesium, however, have completed subshells and are grouped with the stronger electropositive alkali metals which appear in group l-A of the periodic table. This group includes the elements hydrogen, lithium, sodium, potassium, rubid- Patented Oct. 18, 1960 ium, cesium, and francium. Of this group, hydrogen isnot a metal nor a sensitizer; francium is a radioactive element and therefore not applicable for ordinary sensitizing purposes; and the alkali metals, lithium, sodium, and potassium, are not known to have the required sensitizing properties. Accordingly, there are no elements in this group other than rubidium and cesium which have sensitizing properties.

In accordance with this invention, an emulsion, such as a boiled, ammonia, or dispersion type silver halide emulsion, is prepared by standard techniques up to the time of after-ripening, and a rubidium or cesium salt in aqueous solution added during the after-ripening period or prior to coating. Thereafter, the emulsion is processed by coating and drying in the manner well known in the art to give an emulsion with increased speed and good contrast.

Preferably, rubidium and cesium are added as iodides, i.e., rubidium iodide and cesium iodide, particularly when dealing with an emulsion containing silver bromide. This is significant in carrying the sensitizer in to the grain.

The iodide anion easily and rapidly replaces the bromide anion adsorbed to the grain surface and carries the rubidium or cesium with it. For example, the rubidium iodide ionizes to form Rb+ and 1- and, because of their relative solubilities, the 1- interchanges for the Br" at the surface of the grain, thereupon making an easier attachment point for the rubidium ion. Other water-soluble salts of rubidium and cesium may be used to good effect, including chlorides, bromides, hydroxides, and carbonates. I

Standard methods of preparing boiled and ammonia silver halide emulsions include the formation, ripening, and after-ripening of the silver halide salts under controlled conditions according to stage of preparation. The first phase in the preparation of a photosensitive material is precipitation of the sensitive salts, which is usually done in a colloid solution. However, in the case of a dispersion-type emulsion, there may be no colloid present. Interrelated factors, such as temperature, solution concentration, precipitation rate, stirring rate, and presence of silver halide solvents, determine grain size distribution and the characteristics of the photographic emulsion and may be varied as desired. The second phase of photosensitive emulsion preparation involves the ripening or aging of the silver halide precipitate, which produces a shift in its grain size distribution and is governed by temperatures, silver halide content, solvent, stirring, and colloid concentration. The third phase is after-ripening during which final sensitivity is achieved. This phase is also influenced by temperature, bromide concentration, type of gelatin, stirring, and pH.

The invention will be described in greater detail in the following examples, which are presented for purposes of illustration and not as indicating the limits of the invention.

Example 1 Solution 1 is formed by adding 3.45 grams of colloid (such as gelatin, gum arabic, sodium carboxymethyl cellulose, and polyvinyl alcohol plus urea) to milliliters of water. Solution 2 is formed by adding 31.0 grams of potassium bromide and 0.75 gram of potassium iodide to 100 milliliters of water. Solutions 1 and 2 are then mixed and heated to F. Solution 3 is formed by adding 20.8 grams of silver nitrate to 122 milliliters of water. This solution is heated to F. and metered in 25 minutes into solutions 1 and 2,v representing the first precipitation stage. The mixture is maintained at 140 F. for 70 minutes, the first ripening stage, and then heated to 160 F. Solution 4is formed by adding 17.6 grams of silver nitrate to 101 milliliters of water, and this solution is heated to 160 F. and metered in 1% minutes into the mixture of solutions 1, 2, and 3 for the second precipitation stage. Solution 5 is formed by adding 18.6 grams of colloid to 80 milliliters of water, and this solution is heated to 160 F. and added at that temperature to the former combined mixture, and the entire mixture is held at this temperature for minutes for the second ripening.

To the above boiled-type control emulsion is then added the desired concentration of rubidium hydroxide in aqueous solution, such as 3.15 X 10- mol percent of rubidium hydroxide based upon mols of silver halide present in the emulsion. The mixture is then after-ripened to bring it to its maximum light sensitivity and coated on plates at different time intervals in the conventional manner. The plates are then dried, developed, and evaluated sensitometrically. Addition of rubidium hydroxide to the emulsion increases the sensitivity, and therefore the speed, of the emulsion without excess fog, the increase in sensitivity varying according to the concentration of rubidium hydroxide employed. Thus, in comparison to a control aliquot having speed of 14.0, gamma 1.28, and fog level 0.16, various concentrations of rubidium hydroxide when plotted against highest speed attainable exhibit the bellshaped curve typical of sensitizer effects, in concentrations ranging from 3.15 X 10- mol percent to 3.15 10- mol percent rubidium hydroxide based upon mols of silver halide present. These concentrations give speed increases of from 3.5 to 15.0 points above the speed of the control emulsion while gamma is from about 1.00 to about 1.40, and fog level from about 0.18 to 0.50. Other factors ,such as extent of after-ripening time, stirring, etc., may influence the effects of the sensitizer and its optimum concentration under different conditions and with different types of emulsions, but in this particular example the sensitivity effects of rubidium hydroxide achieved speed increases of more than double that of the control at an optimum concentration, with lesser speed increases for concentrations on either side of the optimum concentration.

Example 2 Solution 1 is prepared by adding 33.3 grams potassium bromide, 1.13 grams potassium iodide, and 7.5 grams of gelatin to 252 milliliters of distilled water. Solution 2 is prepared by adding 37.8 grams silver nitrate and 51.1 milliliters of concentrated ammonia to 126.2 milliliters of distilled water. Solutions 1 and 2 are heated to 104 F., and solution 2 added to solution 1 through a metering orifice in six minutes. The resulting precipitate is then ripened for 50 minutes at 122 F. Solution 3 is prepared by adding 35.35 grams of gelatin to 70.7 milliliters of distilled water. It is heated to 113 F. and then added to the mixture of solutions 1 and 2. The mixture is stirred for 10 minutes at 122 F. The gelled emulsion which forms is then chilled and maintained at 20 F. for 24 hours, after which it is noodled and the noodles washed to an E value of 500. The E Ag value is then adjusted to 550 with 0.1 N potassium bromide, and the pH value adjusted to 7.0 with 0.1 N sulfuric acid. Previously swelled gelatin is then added in the amount of 55 grams per kilogram of emulsion.

To the above part-ammonia-type control emulsion is added a desired concentration of cesium chloride in aqueous solution, such as 3.0Xl0'- mol percent of cesium chloride based upon mols of silver halide present in the emulsion, and the mixture after-ripened and coated on plates. As against a control emulsion having a speed of 36.5, gamma 0.80, and fog level 0.12, cesium chloride (in aqueous solution, the amount of Water present not being significant except that there should not be enough water to dilute the emulsion too much, as will be apparent and well known to anyone versed in the art) in concentrations ranging from 3.0 10- to 3.0 10- mol percent-cesium chloride to provide speed increases in the emulsion of from 11.0 to 173.5 points while gamma is from 0.70 to 4 1.12 and fog level from 0.13 to 0.40. Other influencing factors mentioned previously would determine optimum concentrations in specific cases, and various concentration amounts would give varying speed increases according to their relation above or below the optimum amount but from comparison with the control it is evident that cesium chloride has a strong sensitizing died without creating any appreciable fog increase, since in this example an optimum concentration gavea speed increase of more than two and one-half times the speed of the control emulsion.

Example 3 A third type of emulsion capable of being sensitized by rubidium and .cesium salt additions is the re-dispersion type, methods of preparation of which are described in our co-pending application for United States Letters Patent, Serial No. 467,934, filed November 10, 1954, Manufacture of Photographic Emulsions. Thus, one method of preparing this type of emulsion would be as follows: In 200 milliliters of water, containing perhaps a trace of gelatin, 31 grams of potassium bromide and 0.25 gram potassium iodide are dissolved. 38.4 grams of silver nitrate are dissolvedin 223 milliliters of water, one milliliter of the silver nitrate solution being subsequently added to the halide salt mixture and allowed to seed the solution with crystallization nuclei. The rest of the silver nitrate solution is added in a rapid pour after seeding has been completed, this precipitation being performed at F. After settling of the precipitate and thorough washing by decantation or centrifuging, the precipitate is redispersed into a colloid solution, for example, gelatin, with acoustical energy and with or without the aid of a wetting agent. A silver-halide solvent, such as ammonium hydroxide, is added to the emulsion after homogeneous dispersion has been obtained, and then the solvent removed or inactivated, i.e., by the addition of a neutralizing acid such as citric acid, in the case of ammonia. The precipitate and bromide ion concentration are then adjusted to the optimum condition for after-ripening, in accordance with the procedures of Example 2.

Addition of rubidium carbonate in desired concentrations is made to the above redispersion-type emulsion, in aqueous solution, i.e. 2.15 X 10- mol percent of rubidium carbonate based upon mols of silver halide present in the emulsion, the mixture after-ripened and coated on plates. As against a control emulsion having a speed of 20.5, gamma 1.13, and fog level 0.38, rubidium carbonate additions at this stage in concentrations ranging from 3.15 10- mol percent to 3.15 10- mol percent provide speed increases of from 3.5 to 23.5 points above the speed of the control, gamma and fog level remaining consistent with control figures, i.e., gamma from 1.10 to 1.28 and fog level from 0.24 to 0.34. Sensitizing effects of rubidium carbonate also showed the typical bell-shaped curve of diminishing sensitivity on either side of the optimum concentration point, although the optimum concentration amount would be expected to vary, as has been described, in emulsions using different gelatins, or prepared under varying temperature conditions, or which are influenced by stirring, pH, etc. In this case, the addition of the sensitizer in an optimum amount produced a speed more than double that of the control.

It is obvious thatvariations and modifications may be made from the foregoing illustrative embodiments without departing from the spirit of the invention. For example, methods and amounts of materials used in the preparation of the various types of emulsions may differ, so long as these sensitizers, the water-soluble salts of rubidium and cesium, are added in aqueous solution during after-ripening or prior to coating.

What is claimed is:

1. In a process for preparing a sensitized photographic silver halide emulsion, the steps which comprise adding to a ripened washed silver halide a speed sensitizing amount of a water-soluble salt selected from the class consisting of the water-soluble salts of rubidium and cesium, with the sole metallic compounds present during said process being salts of silver and water-soluble salts of alkali metals, and thereafter after-ripening the resulting composition.

2. A process in accordance with claim 1 in which the water-soluble salt is a water-soluble salt of cesium which is present in the amount of 3.0X10" to about 3.0 10 mol percent based upon the silver halide content.

3. A process in accordance with claim 1 in which the water-soluble salt is a water-soluble salt of rubidium which is present in the amount of 3.15 X10" to about 3.15 10- mol percent based upon the silver halide content.

References Cited in the file of this patent UNITED STATES PATENTS 2,038,307 Morrison Apr. 21, 1936 2,399,083 Waller Apr. 23, 1946 2,448,060 Smith et al. Aug. 31, 1948 2,709,134 Jacobs et a1 May 24, 1955 OTHER REFERENCES Phot. Abstracts, vol. 36, #3,Oct. 31, 1956, p. 138. 

1. IN A PROCESS FOR PREPARING A SENSITIZED PHOTOGRAPHIC SILVER HALIDE EMULSION, THE STEPS WHICH COMPRISE ADDING TO A RIPENED WASHED SILVER HALIDE A SPEED SENSITIZING AMOUNT OF A WATER-SOLUBLE SALT SELECTED FROM THE CLASS CONSISTING OF THE WATER-SOLUBLE SALTS OF RUBIDIUM AND CESIUM, WITH THE SOLE METALLIC COMPOUNDS PRESENT DURING SAID PROCESS BEING SALTS OF SILVER AND WATER-SOLUBLE SALTS OF ALKALI METALS, AND THEREAFTER AFTER-RIPENING THE RESULTING COMPOSITION. 